In a typical telephone network, two types of echoes may be present: acoustical echo and electrical echo. Acoustical echo is susceptible to occur in a telecommunication network when a hands-free telephone terminal is used. The speech signal generated from the speaker in the terminal propagates in the form of an acoustic wave through an acoustic environment (air) and part of it will be fed back toward the microphone of the terminal. This signal will be transmitted back to the talker and therefore creates echo.
Electrical echo results from the presence of a hybrid converter that is required to connect the unidirectional four wire link from the public switched telephone network (PSTN) to the local two wire loop. The basic function of the hybrid converter is to separate the transmitted signal originating in the local loop from the received signal in the PSTN section, and vice versa. This process requires the energy of the received signal to pass fully in the local loop. However, due to an impedance mismatch in the hybrid converter, part of the received energy is reflected back to the transmitting port. As a result, a talker hears his own delayed speech which, of course, is undesirable.
The traditional approach for reduction of echo in communication networks is to use echo suppressors. A typical echo suppressor acts like a switch that monitors the voice signals travelling in both directions. It detects which person is talking and blocks the signal travelling in the opposite direction. The drawback of such echo suppressor is that they tend to "chop" speech signals when the subscribers talk back and forth quickly due to the response time for monitoring the speech activities. Moreover, during double talk, i.e., when the subscribers talk simultaneously, the suppressor fails to control the echo.
One possibility to avoid the problems of echo suppressor is to provide circuitry or an algorithm that, instead of blocking speech signals in one direction in the communication link, cancels the echo by using an adaptive filter. In essence, an adaptive echo canceller synthesises the echo that is then subtracted from the composite signal (speech signal plus echo signal). Residual echoes from this subtraction can still be at noticeable levels in a practical echo cancelling system because the adaptive filter may not be able to model perfectly the true echo path as a result of time-varying room impulse response, insufficient filter length, non-linear effects, finite precision computations, etc. Consequently, a non-linear processor, such as a centre clipper, is often used in an echo canceller for further reduction of the residual echo. However, improper design of such devices may cause speech clipping or introduce clipping sounds that interrupt the communication.
Thus, there exists a need in the industry to provide an improved echo canceller, particularly well suited for use during a communication session involving at least one hands-free telephone terminal or a communication network comprising a hybrid converter.